Apparatus for casting by the lost wax process



B. WElssMAN May 30, 1967 APPARATUS FOR CASTING BY THE LOST WAX PROCESS Filed March 1, 1965 United States Patent Otlce 3,322,187 APPARATUS FOR 'CASTNG BY THE LOST WAX PROCESS Bernard Weissman, 3406 Poplar St., Oceanside, N.Y. 11572 Filed Mar. 1, 1965, Ser. No. 435,862 11 Claims. (Cl. 164-376) This invention relates to an improved method of casting by the lost wax process and to a novel apparatus particularly adapted for practicing the same.

The lost wax process has found considerable application in the dental, jewelry and allied elds where the precise reproduction of an article as represented by an expendable pattern is desired.

Essentially, the lost wax casting process involves the formation of a pattern of the desired object to be cast. The pattern is customarily formed of wax or plastic having the desired burnout characteristics. The wax pattern, to which sprues of the same material as the pattern have been attached, is then embedded in a mixture of refractory investment materials. The resulting invested pattern is then subjected to intense heat in order to drive out moisture from the investment material and to completely eliminate the wax or plastic used for the pattern and sprue. The burnout procedure results in the formation of a mold cavity in the investment mass. Molten metal is then introduced into the mold cavity by pressure or centrifugal casting methods and the resultant cast is recovered by destruction of the investment mass. The procedure outlined is almost universally employed for the casting of dental appliances from precious metals, and consequently the instant invention will be more particularly described with reference thereto. 'It will be understood however, that the invention is equally applicable to the jewelry and other fields, where casting is employed.

It is therefore an object of this invention to provide an improved method and apparatus for practicing the lost wax casting process whereby the preparation and mounting of the pattern being utilized is considerably simplified and rendered more efficient, as is the casting process, and wherein an improved cast product is produced.

Another object of this invention is to provide a method for practicing the lost wax casting process, wherein the burnout procedure is rendered more emcient and less susceptible to the possibility of generating undesired gases or residues which may adversely affect the casting process and render the cast product imperfect or unacceptable.

A further object of this invention is to provide an apparatus whereby the foregoing and other benefits and advantages in practicing the lost wax casting process may be achieved.

Other and further objects, benets and advantages of this invention will become apparent from the specications and claims appended thereto, as well as the drawing wherein:

FIGURE 1 is a perspective View of a novel apparatus, herein designated as a casting pedestal, particularly adapted for practicing the instant invention;

FIGURE 2 is a sectional view thereof taken along line 2-2 of FIGURE l;

FIGURE 3 is a bottom plan view of the casting pedestal as viewed in the direction of the arrows 3-3 in FIGURE 2;

FIGURE 4 is an exploded view, in perspective, showing a plurality of patterns for dental appliances mounted upon the crucible base portion of the casting pedestal and showing a fragment of a casting ring about to be 3,322,187 Patented May 30, 1967 lowered in position on said base in the direction of the arrow;

FIGURE 5 is a longitudinal sectional view of a group of patterns in invested form disposed in a casting ring and carried by a casting pedestal in accordance with the preferred form of the present invention;

FIGURE 6 is a View, similar to FIGURE 5 showing the mold cavities formed in the investment mass after burnout of the wax pattern has been. accomplished;

FIGURE 7 is a view similar to FIGURE 6 after metal has been cast into the mold cavities; and

FIGURE 8 is a perspective View of the group of dental appliances with sprue forms still attached, after having been removed from the investment mass.

FIGURES l and 2 illustrate a preferred form of apparatus for practicing the instant invention. The device comprises what is herein generally designated as a casting pedestal 10. Said casting pedestal 10 comprises an annular crucible base member 12 which is provided with a circumferentially disposed depressed portion in the upper face thereof, defining an annular groove or seat 14 for locating and receiving the lower end face of the cylindrical casting ring 22, as may be more clearly seen in FIGURES 4, 5 and 6.

The crucible base member 12 of the casting pedestal 1i) comprises a generally disk-like element which is further provided with a centrally disposed protuberance 20, which defines a cavity in the undersurface thereof, as indicated by the numeral 21. The crucible thus defined in the base member 12 by the inner wall of the protuberance comprises a concavity which in longitudinal. section has ogival or parabolic characteristics. The protuberant portion has also formed therein a cylindrical depression or socket 19 for receiving and retaining the generally cylindrical core member 1.8 of the casting pedestal, as may be seen in FIGURE 2.

A plurality of sprue apertures 16 extend through the protuberance wall to provide communication with the crucible cavity 21. These apertures are spaced from and disposed annularly about the base of the core member 18 and the socket provided therefor in the base member. The axis of each of said apertures 16 is disposed at an acute angle with respect to the longitudinal axis of the pedestal being directed generally to a common focal point within the generally parabolic crucible cavity 21 and 1ocated along said axis.

T-he crucible base and core members are formed of a porous material advantageously having a higher degree of porosity than the investment material utilized and being capa-ble of withstanding the requisite manipulation, the high tempe-ratures involved in the burnout and casting process. Of special advantage is: the use of charcoal Afor this purpose. The members comprising the casting pedestal may be cut from a charcoal block and then assembled or the pedestal may be formed as a unit by other suitable means. It will be understood that many of the advantages of this invention may be realized using either member alone. The highly porous and active character of the charcoal makes it particularly desirable for the accomplishment of the objects of this invention, as will more particularly be .hereinafter pointed out.

In the lost wax processing as used in the dental and jewelry art the initial step is the formation of an accurate pattern of the object desired. In the case of dental appliances this may comprise an inlay or crown. The pattern is formed of wax although patterns formed of plastic materials having suitable burnout characteristics may be employed. Such patterns are illustrated in FIG- URES 4 and 5 of the drawings. A sprue channel forming rod 24 of a similar material is then attached at suitable points to the pattern, as is well known in the art. The

sprue rods 24 are attached for the purpose of ultimately forming a sprue aperture and channel through which the molten metal will flow and feed into the mold cavity to form the desired casting. It has been considered essential to form sprues in `such manner that they lea-d into the mold cavity through the shortest route and to shape them so as to reduce turbulence as much as possible in the flow of the molten metal. A very significant Source of difficulty in casting is the entrapment of gases in the sprue channel or mold cavity before they have a change to escape. Sprue channels presenting sharp turns cause considerable turbulence which entraps gases and so leads to faulty castings. The undesired effects of unavoidable turbulence are reduced, if not eliminated, by the improved venting provided by the instant invention. Although meticulous and time consuming attention to these details by the skilled may reduce turbulence and gas entrapment, it cannot be eliminated entirely and may result in unsatisfactory castings, particularly in the hands of those not highly skilled. The use of the instant casting pedestal of porous active charcoal `greatly facilitates the removal of gases from the mold cavity and reduces, if not completely eliminates, the problem of entra-pped gases, as will be more particularly hereinafter pointed out.

The wax patterns 26 with the sprue forming rods 24 attached thereto are mounted yupon the base member 12 in the manner illustrated in FGURE 4 of the drawings. As is there shown, the free end of each of the sprue forming rods 24 is inse-rted into one of the apertures 16 and may ibe suitably waxed into position. The wax patterns and these attached sprues are thus grouped about the charcoal core 18, spaced therefrom, but in general alignment therewith. The sprue forming rods 24 are advantageously disposed at a small angle with respect to the axis of the core, being divergent therefrom in the direction away from the crucible base member 12. As many patterns as desired may thus be mounted. The limiting factor being the number of apertures 16 available. It will be noted that by means of the instant arrangement the sprues and patterns are almost automatically centered in the casting ring to be employed. The symmetrical grouping which is readily attainable by means of the instant invention greatly facilitates the investment of the patterns permitting the use of thin investment walls without breakthrough and causing each of the patterns to be subjected to a uni-form degree of heat during burnout and casting.

A casting `ring 22 of conventional form, comprising a hollow metal cylinder such as is employed in the lost wax process is now disposed on the crucible base. The lower edge of the ring 22 is located by and received upon annular seating groove 14. It has heretofore been considered essential to line the casting ring with a layer of Wet asbestos. The purpose of the asbestos has been to provide room for the setting and thermal expansions of the investment materials. Although such asbestos lining may be employed with advantage with the instant invention, it is not shown here since it has been found that in many instances such lining may be dispensed with when utilizing a charcoal core in accordance with the present invention. It appears that a core of this type permits the expansion of the investment material radially inward instead of radially outward. Since the mold cavity is enlarged by expansion of the investment material and compensates for metal shrinkage when the cast is made, the use of an asbestos lining with the instant invention may be particularly desirable in some instances. The simultaneous expansion of the investment in all directions, i.e. radially inwardly as well as radially outwardly, reduces the tendency of the mold cavity to become distorted and a more accurate casting may thus be produced.

With the casting ring in position as indicated in FIG- URE the investment material is poured in accordance with accepted practice and envelops the sprued patterns,

lling the casting ring. It shoul-d be noted that the level of the investment material is maintained below the level of the upper or `free end of the core member 18, which is thus exposed to the atmosphere.

It should be noted that by means of the use of the charcoal casting pedestal 10 there has been provided a plurality of wax patterns and the associated sprues, about a centrally disposed charcoal core which extends axially through the ring enclosed investment mass and that there is thus provided a shorter and more highly porous path for the escape of gases from the mold cavities than that ultimately provided by the( investment material. The entire pedestal assembly is capable of withstanding manipulation at the temperatures involved.

Conventional hardening and burnout procedures may now be followed. The burnout of the investment covering the sp-rued patterns drives off the moisture and eliminates the wax or plastic used. In the usual practice, great care and control must be exercised to avoid moisture and other residues, as well as the formation of excessive amounts of sulphur gases due to the breakdown of the calcium .sulphate in the investment. Such gases readily contaminate the gold used in dental casting, rendering it weak and brittle. Heretofore, the sole path for the escape of moisture and gases has been through the ends of the cylindrical investment mass. Consequently, the venting of the center of the mass through the longer path has been inefficient and has made it necessary to utilize higher temperatures which often results in carbonization `of the Wax and breakdown of the investment material into -undesired gases. The provision of the porous casting pedestal in accordance with the present invention greatly facilitates complete and efficient burnout, providing a path for the removal of all undesired products of the burnout process and subjecting them to the reducing action of exposure to charcoal surfaces as they pass therethrough.

The center core provides a more porous path for the escape of gases and moisture from the hitherto remote areas and further provides a path for the introduction of heat to the very center of the mass. There is a more equal thermal distribution throughout the mass which in addition to facilitating burnout also A'appears to result in more sound cavity walls. The pattern Wax melts out more quickly from the hitherto remote portions of the investment mass and at lower applied temperatures thereby preventing carbonization and formation of undesired carbon residues. This result is achieved largely by the inclusion of the axially disposed porous core. It should also 'be noted that venting at the ends of the cylinder is not affected due to the porous character of the charcoal base member. Upon completion of burnout, as shown in FIGURE 6, void areas designated generally by the numeral 40 are formed. These hollow areas comprise the mold cavities 30 in communication with the sprue channels 28.

The burned out investment now forms a mold which comprises a network of pores or interstices through which hot gases are also to be vented during the casting process.

Casting is generally carried out by the centrifugal process well known in the dental and jewelry art. The molten metal, which in the dental art may be gold, platinum, iridium or chrome cobalt alloy, is forced into this mold cavity 30 through the sprue channels 28 of the burned out mold, which is disposed horizontally during this process. It will now be noted that in accordance with the instant invention the molten metal is initially thrown or fed into the crucible cavity 21 of the crucible base 12. The generally parabolic conformation of the cavity walls provides a smooth uninterrupted path for the non-turbulent ow of molten metal into the apertures 16 in communication with the sprue channels 28. The apertures being symmetrically disposed provide for an even distribution and flow of the molten metal into the sprues. In view of the fact that the base is formed of charcoal, the metal is immediately subjected to the reducing action thereof, tending to reduce or eliminate oxides from the metal. This action continues as the molten metal progressively enters the sprue channels and the interface of the molten metal with the charcoal changes. The molten metal is further subjected to the reducing action of the charcoal with which it comes into contact as it passes through the constricted passages defined by the walls of apertures 16. The porous and active character of the charcoal prevents the formation of entrapped `bubbles and permits the initial escape of occluded gases. The molten metal is thus purified and conditioned 'by the contact with the charcoal surfaces and a smoother and more rapid tiow of the metal is accomplished. The depth of the concave cavity 21 further provides a chamber within which the metal is maintained in molten state for a longer period of time to permit the mold cavities and sprues to be completely filled.

As the molten metal enters the sprue channels 28 the gases being displaced must be vented as quickly as possible. Although the investment material is porous and now constitutes a network of interstices, these are relatively narrow and the path to the cylinder and faces is relatively long. Considerable friction is encountered along the walls of the interstices. A yback pressure may thus be built up causing turbulence in the metal, entrapment of gases and possibly some fragmentation of the investment material in the sprue or cavity walls. The flow of metal may be reduced, preventing the mold cavities from being filled. In yaccordance with the present invention however, a relatively short path to the more highly porous core member 18 is provided immediately as the molten metal -begins to enter the aperture 16 and the sprue channel 28. As the molten metal enters aperture 16, additional and even more immediate venting of gases through the short and more highly porous path is provided through the material of the -base mem-ber itself. The smooth continuous flow of metal is thus unimpeded throughout the casting process and the extent to which the gases must pass through the more restricted interstices of the investment material is minimized and generally equalized for all portions of the sprue and mold cavity alike. The mold cavity is thus completely filled and the rounding off of sharp edges and corners of the cast due to the formation of gas pockets is prevented. The metal is fed smoothly and continuously from the -button 36 and there is no backtiow of molten metal which may result in a porous or hollow casting.

FIGURE 7 illustrates the complete cast 34 as it appears within the investment mass 32 and FIGURE 8 illustrates the casting after removal therefrom, the sprues being indicated by the numeral 24C and the cast dental appliances `by the numerals 26C. It has been found that the castings produced by the instant method and app-aratus are sound without porosity or hollow areas. That Vall details are sharply defined. The castings present smooth bubble and blemish free surfaces. In practice it has been found that the surfaces produced are suciently smooth and of such high quality that a highly lustrous surface may be produced by simple bufing.

The casting pedestal, particularly the porous charcoal core member provides for improved heat exchange which in addition to permitting uniform heating of the mold also provides for more rapid and uniform cooling thereof.

It will be understood that there has here -been described a preferred form of the invention and that changes and modifications thereof may be made without departure from the spirit and scope thereof.

I claim:

1. Apparatus for improving the Iburnout and casting characteristics of a refractory investment mold, said apparatus comprising a crucible base member formed of a gas pervious material which is capable of withstanding the mold processing temperatures involved and is adapted to be disposed along one end of said mold, said -base member comprising a disklike element provided with a concave cavity in one face thereof and at least one -aperture extending through said base member and in communication with said cavity, said one face of said element being free of investment material and being exposed to the atmosphere, thereby providing a path for heat exchange with the investment mass and for venting of gases therefrom.

2. Apparatus according to claim 1 wherein said crucible base member is formed of a material having a higher degree of porosity than the refractory investment material of the mold.

3. Apparatus according to claim 1 wherein said cruci- =ble lbase member is formed of a porous carbon.

4. Apparatus according to claim 1 wherein said crucible base member is formed of charcoal.

5. Apparatus according to claim 1 including a core member formed of a porous material wherein said crucible base member supports said core member.

6. Apparatus for facilitating the formation and use of a mold formed of a mass of refractory investment material and for improving the quality of the casting derived therefrom, said apparatus comprising a casting pedestal formed of a porous material which is capable of withstanding the mold processing temperatures involved and including a generally disklike crucible base member adapted to overlie one end of the investment mass comprising the mold and a core member projecting from the central portion of said -base member and being adapted to extend through said investment mass to provide a porous core therefor, whereby said mold is vented and the heat transfer characteristics thereof are improved.

7. Apparatus according to claim 6 wherein said casting pedestal is comprised of porous carbon.

8. Apparatus according to claim 6 wherein said casting pedestal is formed of charcoal.

9. Apparatus according to claim 6 wherein said base mem-ber is provided with a plurality of apertures for communication with sprue channels formed in the mold.

10. Apparatus according to claim 6 wherein said base member is provided with a centrally disposed protuberance on one face thereof, said protuberance providing a support for said core member and defining a crucible cavity in the opposing face thereof, said protuberance being provided with a plurality of spaced apertures disposed about said core member in spaced relation thereto, said `apertures being in communication with said crucible cavity.

11. Apparatus for facilitating the processing and use of an investment casting mold as used in the lost wax casting process wherein a pattern formed of wax or plastic having -a sprue forming rod attached thereto is surrounded 'by a casting ring and enveloped by a refractory casting material, the `burnout of said pattern and sprue forming rod resulting in the formation of a mold cavity having a sprue channel wherein molten metal may be introduced to Iform a casting corresponding to said pattern, said apparatus comprising a porous casting pedestal including a cruci-ble base member and a core member supported by said base member disposed substantially centrally thereof, said base member comprising an annular disklike formation having a centrally disposed protuberance and a circumferentially disposed ring seat on one face thereof, said protuberance defining a crucible cavity of generally parabolic conformation in the opposing face of said base member, said protuberant portion being further provided with a socket for receiving one end of said core member and a plurality of apertures disposed about said socket portion and extending through said protuberance and in communication with said crucible cavity,l the axes of said apertures -being inclined at an acute angle with respect to the axis of said pedestal, said apertures being adapted to receive and support sprue forming rods.

References Cited UNITED STATES PATENTS Lanier 22-97 X Palmer 2297 Williams 22-97 McCann 24S- 44 Siler 22-13 8 Williams 22-171 X Mackett 22-134 Parlanti 249--62 Lirones et al 22-196 I. SPENCER OVERHOLSER, Primary Examiner. E. MAR, Assistant Examiner. 

1. APPARATUS FOR IMPROVING THE BURNOUT AND CASTING CHARACTERISTICS OF A REFRACTORY INVESTMENT MOLD, SAID APPARARATUS COMPRISING A CRUCIBLE BASE MEMBER FORMED OF A GAS PERVIOUS MATERIAL WHICH IS CAPABLE OF WITHSTANDING THE MOLD PROCESSING TEMPERATURES INVOLVED AND IS ADAPTED TO BE DISPOSED ALONG ONE END OF SAID MOLD, SAID BASE MEMBER COMPRISING A DISKLIKE ELEMENT PROVIDED WITH A CONCAVE CAVITY IN ONE FACE THEREOF AND AT LEAST ONE APERTURE EXTENDING THROUGH SAID BASE MEMBER AND IN COMMUNICATION WITH SAID CAVITY, SAID ONE FACE OF SAID ELEMENT BEING FREE OF INVESTMENT MATERIAL AND BEING EXPOSED TO THE ATMOSPHERE, THEREBY PROVIDING A PATH FOR HEAT EXCHANGE WITH THE INVESTMENT MASS AND FOR VENTING OF GASES THEREFROM. 